Production of electrolytic chromium from ferrochrome



Patented Dec. 22, 1953 MIUM F301) 'Rex- R. Lloyd, Joe-BtRosenb'aum; andVernon .:Hom. ne,'B9 .u det Gi w, ass e exti e .ll eistates 9f America:renre ented by -(the s li its bf i 1 ren es-. es eieriqr e l eii e WW8fii ikserie N9, ?.-?$i$6 s'clain (01. Zo a-19 5 (Grant d under Title 35;ga G94? (1952), see. 266)" -r z 1 a e inventioridescrihed hereinbe'lmariu f actured and ,usedby or ior thelG' mm f the United States forf bvernment purposes without the payment tous'of any royalty'th'e eoifiinfacc ordance with the provisions ,ofitiie Act of .Ap'ri1 30,f1928 (on;r50, ,45 'statL. 4 67)";

"firms appn'catmn'relates t6 he pjr diiqtiori of electrolytic jehremiumri "rerrophro 'e and mor particularly" to awarene s o; atta in anifectrolytef produced' jgirom' rerreeurome' or an electrolytic cell fromwhiehpureerirqmium metal qhm i vm weieie is to be obtained; I x Thisapplicaflion is relatedlto copendmg appl ique r t uesior iron entitledElectrolytic Dianhragin Qell; 1 1 238,351, ,fil'ed July 24, 19 51; Ldiandve i --'H I 'mme en le 'Ei amdepositm b.2- Ql lromium andElectrolyte rnererqr'xwerm no. 238,360, m d Ju1y'24 1951i: annex 3- loydand Joe B. Rqsenba m entiuedf xno r rEiejetrqfwinnin "chro ium.m taupservno. 5,363, filed 111131.24, 19,51. 1 j "'Ch' czm m 'meta lu i e tweos si om narily'jproces sed .by smelting; es which jcgnt ain alhi gh'such asthree to Q'nefrat ,Qf ChrQIni'ur'n'to iron in an electric furnag etjvitha 'carbo ed c n a n to" pl bduee ca to me- ,t 'o cert ie Purp es sash aspi ti n 9 s a nless st eluc rhqnicaninbt be It Nearly all domes ic presfo ei er, (do m tain a fillt cientily iigh raj ioofc produceferro'lchrom'e", contain'i chromium,l lhiehiisk fi d hii lurgicalpractice. The! pro lem" which led, to 'tl re less costly than-the llieretofor e custo ary pr 1i esses by which electrolytic chronri rmmchrorni-c acid or'loy :the .aluini reduction lof enig a oxide Farmermeets and adtantae ;tlre :1.

v ydonventio'nal and vtirlellikn'onriri' V JP a tdea? ea new a con ierai" an! 'f'lihe figure Of the drawing is afiow sheet oitlrereterreclproicesser th'e'inv icn In one embodimen't'of' rin ne t h dew sit.

reirqth m Ordinarily liigll-carigon iqnoclrrorrre is used because ofcost, but the process is equally applicable to any alloy-containingehromiumand iron which can be ti issolved'in'aeid. "Ferrochromeordinarily contains major quantities of chromium; iron; sili-- con, andcarbon; arid minor quantities' of several other metals 'suchas nickel,cobalt, and Vane)- diiim.

' "According to the invention, fBIIfOQhIOIIlGZiSrdiSF solved 'in" a .hotsolution containing 'sulfur ie acid. As shown in the'fiow sheet, thissolution is prefeerabiy ".t'he"-retu1"n depleted chrome iliq'uor whichis enriched withsufiicient sulfuric acid to-make up for losseswinxtheprocess. This depleted chrome liquor is 'primarily a-s'olutionogfvsulfuric acid-,andit also contains ammonia'or ammonium sulfate andminor :quantities of iron; sulfate and chromium sulfate. The'majorportion of the acid required for the :treatmentxof the .fer-rochromesispresent the depleted chrome liquor andgprerierably, an exeess ofaeidiisused and recycled with the. chrome .Iiquor. 9

It has :been discovered that ferrochr'ome' will not-d'issolve'in'acidili'quor which contains ihexaj .va-lentachromium 'ortrivalent iron.It is, gtherefore; essential thatzthexleach liquor used-to treat theferrochrome be substantially free ofbotli chromium and iron of :these:highfer valences.

"'iWhen' IGII'OChTOmBLiS dissolved in sulfuric acid, the. chromium:is'comierted principally to chromijc sulfateiwith some 'chromous"sulfate flairon :is can'- -ve'reted.to ferrous*suliater someofethezinetallic impurities-1mm enter the solutiondin varions 20felectrolytic 9) temperature of from 60 to 80 C., the latter temperature,80 C., being preferred, for at least twd hours and preferably from 8 to24 hours. This conditioning step at 80 C. functions to maintain thechromium in the solution in the green or hydrolized modifications inwhich forms it has a high solubility and does not form alums, and at thesame time the iron appears to de-hydrolize during conditioning,permitting the formation of ferrous ammonium sulfate-crystals uponcooling the liquor following conditioning.

If the conditioning step is omitted, ferrous ammonium sulfate crystalswill form to only a limited extent, if at all, upon subsequent cooling.

This is particularly true if the ferrochrome leach has taken place attemperatures in excess of -90 C. As stated above, conditioning may beperformed at 60 C. to 80 C. However, at the lower temperatures, chromiumreversion to the violet modification is accelerated, resulting inincreased chromium alum precipitation with the ferrous ammonium sulfatecrystals in the subsequent step of the process. If the leach is carriedout at 80 C. plus or minus C., then separate conditioning is lessnecessary but the conditioning step has, nevertheless, been affected.

Crystallization is accomplished by cooling the conditioned liquor to alow temperature. Preferably, the liquor is cooled to 10 C. or less. Thiscooling causes precipitation of ferrous ammonium sulfate, (NHi)2SO4'F8SO4'6H2O', and a small quantity of chromium ammonium alum,

Impurities, such as nickel, cobalt, and titanium, whichare dissolvedfrom the ferrochrome, are also removed in these crystals. l Thesecrystals are filtered from the liquor, and the thus purifiedliquorsubstantially comprises sulfuric acid, chromium sulfate, and ammoniumsulfate. The purified liquor still contains a small amount of ironsulfate as well as small quantities of any other impurities present inthe leach liquor, but these do not precipitate in the subsequent agingstep and eventually build up in the. system until they do come out withthe ferrous ammonium sulfate crystals formed in the above-mentionedcooling and crystallizing step.

. .The purified liquor thus obtained is heated and held at a temperatureabove normal room temperature, with a temperature of approximately 30 C.being preferred. During this aging, the chromic sulfate is convertedfrom the hydrolized and green modifications, which do.v not form alums,to the violet modification where .alum forms and precipitates. The pl-lof the liquor must be below 1.2 and preferably a range of pH of 0 to 0.5is used. It is essential to the production of an adequately pure chromealum that the iron and other impurities be removed in thecrystallization step at a temperature substantially lower, such as 25 C.or below,.than the tempera ture at which the chrome alum isprecipitated;

-'-'- --'Ihe chrome alum is used for the production chromium asdescribed in the above-mentioned oopending application .Serial No,238,360, filed July 2.4, 1951. Theficatholyte overflow from the cellcontains chromic sulfate, chromous sulfate and ammonium sulfate and is.returned directly to the liquor aging tank. The .anolyte overflowcontains hexavalent chromium, ammonium sulfate, free sulfuric acid and asmall amount of trivalent chromium- This anolyte overflow is treatedwith a suitable reducing agent, such'as sugar, ammonia, sulfur dioxideor' the like,

in order to reduce the hexavalent chromium in the anolyte. The anolyteoverflow, after reduction, is also returned to the liquor aging tank.

These liquors are returned to the aging tank since they are free ofimpurities and they help to dilute the impurity concentration in theaging tank. By impurities is meant any constituent other than chromium,ammonium sulfate or sulfuric acid. As stated above, any of severalwellknown reducing agents, such as sugar, ammonia, sulfur dioxide andthe like, can be used to reduce the hexavalent chromium in the anolyteoverflow. If the anolyte is heated to 250 C. or above in an autoclave,the chromic acid will spontaneously decompose according to the followingreaction:

If sulfur dioxide is used for a reducing agent, very little make-upsulfuric acid is required by the process. Evaporation is, practicallyspeaking, necessary somewhere in the flow sheet to remove excess water.If the anolyte overflow is evaporated to a specific gravity of 1.6 orabove, a large part of the hexavalent chromium separates out as a crudechromic acid, which can be purified by well-known means. If the chromicacid is not removed, the evaporation step can be accomplished elsewherein the flow sheet, such as with the leach step, without changing theessential features of the process.

As shown in the flow sheet, the ferrous ammonium sulfate crystals may berecrystallized to recover the chromium or they may be discarded.Conditioning, in the flow sheet, means heating the solution to about C.to stabilize the chromium in the green modifications so that it will notsubsequently precipitate with the iron, The final iron crystals may betreated by conventional methods to recover ammonia or ammonium sulfate,for return to the system as chrome liquor, as shown in the flow sheet,or they may be otherwise utilized or discarded without changing theessential features of the process.

It will be appreciated that the procedure of recycling of solutions inthe novel manner disclosed herein improves the economics of the processby minimizing the losses and by accelerating reactions through controlof chemi-' cal compositions. Thus the catholyte overflow is returned tothe process to prevent the loss of chromium and ammonia; the treatedanolyte overflow is returned in order to save its chromium, ammonia andacid. In addition, return of these highly pure liquors serves to dilutethe impurity concentration in the chromium aging liquor, thus reducingthe quantity of impurities tending to precipitate with the chromiumalum. The depleted chromium liquor, from which the alum hasprecipitated, is returned to the process in order to utilize its acidcontent for ferrochrome dissolution, its ammonia in raising the ammoniaconcentration of the leach liquor high enough to remove iron in thesubsequent iron crystallization step, and its chromium to increase thechromium' concentration to the range in which precipitation of chromiumalum is rapid during the aging step. Dilution of the chromium agingliquor with return catholyte overflow and reduced anolyte overflow isdesirable to reduce the impurities in the precipitated chrome alum. Itwill be appreciated, however, that applicants process may be operatedwithout the recycling steps without departing from the spirit and scopeof the invention.

ace-am- '5 The following example is illustrative of one particularembodiment ofthe process of the invention.

EXAMPLE Experimental data A ISO-pound batch of minus 4; inch highcarbonferrochrome was leached for 48 hours with 407 gallons of depletedchromium liquor fortified with 80 pounds of 93-per cent sulfuric. acid.The ferrochrome contained approximately 64 percent chromium, 25 per centiron, 6 per cent carbon, and 3 per cent silicon. The depleted chromiumliquor contained 20 grams chromium, 2.2 grams iron, 52 grams ammonia,and 200 grams free sulfuric acid per liter. The leach temperature wasmaintained at 95 to 100 C. and the evaporation was controlled, by meanof a condenser on the leach kettle, to 102 gallons of water.

At the end of the 48 hours th leach was filtered hot through a leadfilter press. The filter press residue weighed 15 pounds Wet andconsisted mostly of silica with a small quantity of carbon and contained2 pounds of chromium and 0.5 pound of iron. The filtrate measured 302gallons and analyzed 6'7 grams chromium, 19 grams iron, 70 grams ammonia(all present as the sulfates), and 155 grams free sulfuric acid perliter.

To the 302 gallons of filtrate was added 69 gallons of chrome liquorobtained by recrystallization of the chromium containing ferrousammonium sulfate from a previous batch. The recovered chrome liquoranalyzed about 22 grams chromium, 4 grams iron, and 62 grams ammonia perliter, all present as sulfates. The 371 gallons of mixed chromium liquorwas then conditioned by'holding at 80 C. for 16 hours.

The conditioned liquor Was next cooled to C. in a vacuum crystallizerwhere ferrous ammonium sulfate and other crystals formed. These crystalswere separated from the purified liquor in a perforate-type basketcentrifuge. The crystals weighed 450 pounds and analyzed about 9 percent iron, 3 percent chromium, and 3 per cent ammoni'a. The purifiedliquor analyzed 62 grams chromium, 3' grams iron, 65 grams ammonia perliter and measured 323 gallons.

The impure crystals were dissolved in 57 gallons of water along with 100pounds of ammonium sulfate and the chromium was stabilized byconditioning at 80 C. for 4 hours. The liquor was cooled to 5 C. and thecrystals separated as in the first crystallization, to yield 300 poundsof crystals and 69 gallons of liquor. The final crystals analyzed 13 percent iron, 0.2 per cent chromium, and 8.5 per cent ammonia. Therecovered chrome liquor analyzed the same as the 69 gallons previouslyadded to the leach liquor and was recycled to the next batch.

The purified chromium liquor obtained from the first crystallization waspumped to a continuously aging thickener containing about 4,000 gallonsof depleted liquor. The aging liquor was held at 32 to 35 C. and thealum crystals which formed were separated in a perforate-typecentrifuge, with the liquor returning to the aging tank. Approximately1,330 pounds of chrome alum were produced from this particular batch.The wet alum analyzed 10.6 per cent chromium, 0.03 per cent iron, and3.4 per cent ammonia. A 407-gallon batch of depleted chrome liquor wasremoved from the aging thickener and. returned to the next batch.

The 1,330 pounds of chrome alum was dissolved in 37- gallons of water at70C. to makea ten feed solution containing about 130 gramsch'r" 1'- um,0.4- gram iron, and 42 grams ammonia per liter. This cell feed was fedto a continuously operating cell having five stainless steel cathodeswith a submerged area of 37.5 square feet; The catholyte. solutionanalyzed 17 gramstrivalent chromium, 23 grams divalent chromium, and-'80grams ammonia per liter. The cell feed was added at such a rate that theanalysis of the catholyte solution remained constant throughout thetest. A current of 3,000 ampereso'n the cell, at a potential of 4.7volts, elcctrodeposited 101 pounds of electrolytic chromium from thecath= olyte solution in 39 hours. The metal analyzed 99.3 per centchromium, and 0.4 per cent iron. Of the 141 pounds of chromium enteringthe cell in thecell feed, 100 pounds were electrod'eposited 11 poundsleft the cell in gallons of catholyte overflow, and 30 pounds in 140gallons of anolyte overflow. Theanolyt'e overflow'analyzed 3 gramstrivalent chromium, 22 grams hexavalent chromitum, 19 grams ammonia, and300. grams free sill:- furic acid per liter. Was added to the anolyte,during the test, to keep thespecific gravity lower than the catholyte.The electrolytic cell operated at 60 C. and ap'H of 2.5 to 2.6 duringthe test.

The catholyte overflow Wasreturned directly to the liquor-aging stepwhere the divalentchrom'rum maintained completely reducing conditions:in the aging tank. If a small quantity of divalent chromium, or otherstrong reducing agent", is not added to the liquor-aging step theferrousiron will oxidize to give a trace of ferric iron which, in turn,adversely affects the rate of: alum formation and the purity;

The anolyte overflow solution was maintained at 60 C. while thehexavalent chromium was re.- duced by adding 11 pounds of sugar; Thereduced anolyte was then returned. to the liquoraging tank.

It will be appreciated from a reading. of the foregoing specificationthat the invention. herein described is susceptible: of various changesand modifications without departing from the spirit and scope thereof.The. invention is, therefore, to be limited only by the scope andspiritasset forth in the appended claims.

What is claimed is: r

1. A process for production of electrolytic chromium from ferrochromecomprising leaching said ferrochrome with a hot acidic solutioncontaining chromium depleted liquor from a subsequent stage of theprocess to thereby form a solution containing chromium sulfate, ammoniumsulfate, and iron sulfate, crystallizing the bulk of the iron from saidsolution in the form of ferrous ammonium sulfate, mixing the remainingliquor with an anolyte efiiuent solution and a catholyte effluentsolution containing chromic sulfate, chromous sulfate, ammonium sulfate,and sulfuric acid, aging the resulting liquor at a pH below 1.2,crystallizing chromium ammonium sulfate from the aged liquor at atemperature higher than that at which said ferrous ammonium sulfate wascrystallized, recycling the chromium depleted liquor to provide asubstantial portion of the acid solution for leaching said ferrochrome,utilizing the crystallized chromium ammonium sulfate with water to forman electrolyte feed solution, electrolyzing said solution in anelectrolytic compartment cell in which chromium is plated from thesolution and in which anolyte efliuent containing hexavalent chromium,am-

About 60 gallons of water monium sulfate, and sulfuric acid andcatholyte efiluent containing chromium sulfate, chromous sulfate andammonium sulfate are produced, reducing the hexavalent chromium of theanolyte effluent to lower valence forms, and recycling said reducedanolyte solution and said catholyte solution for admixture with thechrome liquor from which ferrous ammonium sulfate has been removed.

2. The process of claim 1 in which the hexavalent chromium in saidanolyte solution is reduced by heating the anolyte solution to 250 C.

3. A process for production of electrolytic chromium from ferrochromecomprising leaching said ferrcchroine with a hot acidic solutioncontaining chromium depleted liquor from a subsequent stage of theprocess to thereby form a solution containing chromium sulfate, ammoniumsulfate, and iron sulfate, crystallizing the bulk of the iron from saidsolution in the form of ferrous ammonium sulfate, mixing the remainingliquor with an anolyte eiliuent solution and a catholyte effluentsolution containing chromic sulfate, chromous sulfate, ammonium sulfate,and sulfuric acid, aging the resulting liquor at a pH below 0.5

at a temperature of about 30 0., crystallizing chromium ammonium sulfatefrom the aged liquor at a temperature higher than that at which saidferrous ammonium sulfate was crystallized, recycling the chromiumdepleted liquor to provide a substantial portion of the acid solutionfor leaching said ferrochrome, utilizing the crystallized chromiumammonium sulfate with water to form an electrolyte feed solution,electrolyzing, said solution in an electrolytic compartment cell inwhich chromium is plated from the solution and in which anolyte effluentcontaining hexavalent chromium, ammonium sulfate, chromous sulfate andammonium sulfate are produced, adding a reducing agent to said anolyteefiluent thereby reducing the hexavalent chromium of the anolytesolution to lower valence forms, and recycling said anolyte solution andsaid catholyte solution for admixture with the chrome liquor from whichferrous ammonium sulfate has been removed.

4. The process of claim 3 in which the added reducing agent is sugar.

5. A process for the production of electrolytic chromium fromferrochrome comprising leaching said ferrochrome with a hot aqueoussolution containing sulfuric acid, ammonium sulfate, and chromiumcompounds substantially free of hexavalent chromium and trivalent iron,recycled from a subsequent stage of the process, to thereby form asolution containing chromium sulfate, ammonium sulfate and iron sulfate,filtering the thus formed liquor to remove insoluble constituentstherefrom, cooling the liquor to a temperature of from about C. to C.for a period of two to twenty-four hours to maintain the chromium insolution in a hydrolyzed form and to dehydrolyze the iron in solution,cooling the thus conditioned liquor to a temperature below 25 C. therebycausing precipitation of the bulk of the iron as ferrous ammoniumsulfate crystals, filtering said liquor to remove said crystals, mixingthe remaining liquor with anolyte effluent solution and catholyteeflluent solution containing chromic sulfate, chromous sulfate, ammoniumsulfate and sulfuric acid, aging the resulting liquor at a pH of below1.2 and at a tem perature of about 30 C., crystallizing chromiumammonium sulfate from the aged liquor, recycling the chromium depletedliquor to the leaching step with make up sulfuric acid, utilizing thecrystallized chromium ammonium sulfate with water to form an electrolytefeed solution, electrolyzing said solution in an electrolyticcompartment cell in which chromium is plated from the solution and inwhich anolyte effluent containing hexavalent chromium, ammonium sulfate,and sulfuric acid, and catholyte efilucnt containing chromium sulfate,chromous sulfate and ammonium sulfate are produced, reducing thehexavalent chromium of the anolyte efiiuent to lower valence forms, andreturning said reduced anolyte solution and said catholyte solution foradmixture with the chrome liquor from which ferrous ammonium sulfate hasbeen removed prior to the aging step.

REX R. LLOYD. JOE B. ROSENBAUM. VERNON E. HOME IE.

References Cited in the file of this patent UNITED STATES PATENTS Number

1. A PROCESS FOR PRODUCTION OF ELECTROLYTIC CHROMIUM FROM FERROCHROMECOMPRISING LEACHING SAID FERROCHROME WITH HOT ACIDIC SOLUTION CONTAININGCHROMIUM DEPLETED LIQUOR FROM A SUBSEQUENT STAGE OF THE PROCESS TOTHEREBY FORM A SOLUTION CONTAINING CHROMIUM SULFATE, AMMONIUM SULFATE,AND IRON SULFATE, CRYSTALLIZING THE BULK OF THE IRON FROM SAID SOLUTIONIN THE FROM OF FERROUS AMMONIUM SULFATE, MIXING THE REMAINING LIQUORWITH AN ANOLYTE EFFLUENT SOLUTION AND A CATHOLYTE EFFLUENT SOLUTIONCONTAINING CHROMIC SULFATE, CHROMOUS SULFATE, AMMONIUM SULFATE, ANDSULFURIC ACID, AGING THE RESULTING LIQUOR AT A PH BELOW 1.2,CRYSTALLIZING CHROMIUM AMMONIUM SULFATE FROM THE AGED LIQUOR AT ATEMPERATURE HIGHER THAN THAT AT WHICH SAID FERROUS AMMONIUM SULFATE WASCRYSTALLIZED, RECYCLING THE CHROMI-